Ultrasonic transducers are typically formed with one vibrating surface or a plurality of vibrating surfaces capable of converting electrical energy into mechanical displacements and vice-versa. Because the acoustic pressure produced by such devices obeys diffracting laws, physical parameters such as area, frequency, bandwidth, geometry and surface apodization (weighting) are key factors in transducer design and actually govern the radiating acoustic beam pattern produced by the transducer.
The operation of single area transducers is often characterized by spurious boundary effects, which are manifested by secondary lobes occurring laterally of the main lobe. These effects generally occur when the ratio factor between the Z and the X-Y dimensions does not satisfy a certain value. On the other hand, array transducers require substantially perfect and well controlled angular directivities of the corresponding transducer element apertures in order to produce smooth radiating acoustic beam patterns compatible with the formation of a high quality image. Based on the above considerations, designers of ultrasonic transducers often seek to balance performance with the geometry of the transducer.
To date, piezoelectric array transducers are principally of a bulky design wherein a portion of the piezoelectric material is slotted into narrow independent blocks which are isolated from each other and arranged in side by side relation in the azimuth direction. The piezoelectric material is uniformly poled and is of a thickness that is predetermined to provide the desired resonant frequency. Accordingly, the geometry of elemental transducers is, therefore, essentially determined or set at this initial design stage. Further modification of the geometric parameters set at this initial stage is difficult to effect, and, further, will strongly affect the intrinsic acoustic behavior of the transducer device. Usually, taking advantage of any trade-offs with respect to the geometrical specifications of a transducer involves compromise regarding performance and/or cost.
In the recent years, a new family of devices, commonly called CMUTs (Capacitive Micromachined Ultrasonic Transducers) and using semiconductor capacitive micromachined cavities for producing ultrasound, has appeared on the market. These CMUT devices generally have the advantage that, on one hand, collective manufacturing processes (mass production) can be used in making the devices, and, on the other hand, the devices exhibit a broader bandwidth as compared to piezoelectric assemblies. The basic principles of such a device are quite simple and these principles have been successfully implemented for years in the manufacturing of condenser microphones. However, in capacitive transducers, the transducer is governed by a voltage oscillation over an electrostatic field (bias voltage). This oscillation causes the membranes over the cavity to vibrate and to therefore produce output ultrasonic waves. Conversely, when a pressure force acts on the surfaces of the biased membranes, this results in mechanical bending of the membranes and, thus, in creation of an output voltage oscillation. Both the excitation and receiving voltages are provided by the associated imaging system which remains essentially unchanged, i.e., is very similar to existing imaging systems.
Although inherent drawbacks in such capacitive devices still remain (e.g., drawbacks such as device fragility, a biased voltage requirement, a long prototyping cycle, and high volume production needs) such drawbacks will likely be overcome with advances in the microelectronics and sensors technologies. In any event, CMUT devices exhibit certain unique advantages such as the ability of these devices to be integrated readily with microelectronics for immediate signal processing so as to improve the quality of the received information, and the higher degree of miniaturization that can be achieved using these devices. Thus, it can be predicted that in the near future CMUT devices will outperform conventional transducer technologies.
A number of manufacturing methods for CMUT devices have been developed and are currently well known in the art. A common basic method for manufacturing CMUTs comprises at least the following steps: a silicon substrate is provided with an oxide layer deposited on the surface of the substrate; electrodes are patterned over the oxide layer; a sacrificial layer is then deposited thereon and then photolithographically patterned to define cavities to be created in further steps; a silicon nitride layer is deposited over the substrate; vias are dry-etched, and the membrane is released using wet etching techniques on sacrificial layer; the vias so obtained are then sealed; and finally, an outer electrode is sputtered on the top of the membranes. Reference is made, for example, to U.S. Pat. Nos. 5,894,452 and 5,870,351 to Ladabaum et al and U.S. Pat. No. 5,870,351 to Haller.
Other manufacturing methods exist such as bulk micromachined processes where 3D patterns are etched on layers of silicon and then the layers are bonded together at high temperature, under a vacuum, to form the desired cavities.
Methods for integrating electronics on the substrates of CMUT devices have been developed which use a BiCMOS process or low temperature process. This has made the capacitive transducing devices very attractive for the future development of highly integrated ultrasonic imaging systems. Since the devices are manufactured as silicon components or ICs, the packaging and interconnect aspects of manufacture will advantageously benefit from the most recent developments in these fields so as to keep manufacturing of CMUT devices at the leading edge of the relevant technology.
A method for reducing undesirable interaction between elemental transducers (transducer elements) of an array is disclosed In U.S. Pat. No. 6,918,877 to Hossack et al. In this method, the cross talk between the adjacent transducer elements of an array is measured or calculated, and modified excitation signals, derived from signals relating to the selected element, are then applied to the neighboring elements to interfere with any cross talk and thus reduce the effect of the cross talk. As mentioned in the patent, the method can be implemented in most array designs and is especially well adapted to silicon substrate-based MEMS (Micromachined Electro Mechanical Systems) transducers. Although methods such as those based on coded signals or post-calculated signals can be of suitable efficiency, the implementation of such methods in commercial systems presents another challenge. In this regard, in the aforementioned method, an individual adjustment is required for every transducer or type of transducer in order for the method to work properly. Any variation in cross talk or performance, and any lack of homogeneity between elemental transducers of the array, necessitate the requirement that the system must be recalibrated and fine tuned. Thus, this method disclosed in this patent is obviously better suited to laboratory uses than to industrial applications.
A patent that is more closely related to transducer aperture control and apodization methods is U.S. Pat. No. 6,381,197B1 to Savord et al. This patent discloses a CMUT device that includes a variable gain control for MUT cells which are integrated into the substrate of the device. The patent is principally concerned with the use of integrated electronic control circuitry implemented on a common substrate rather than the CMUT devices, switches or microrelays which are provided as well as the passive components such as resistors or capacitors used to control the bias voltage source for the MUT cells. In one of the embodiments, there is provided another method of gain control for CMUT cells wherein the diameter of CMUT cells can be changed or the distance between the CMUT cells can be varied or a combination of the two approaches can be used. The patent discloses that with respect to a change in cell diameter, the larger the CMUT cells, the higher the acoustic energy provided. Unfortunately, when this approach is applied to circular shaped CMUT cells as disclosed by Savord et al. The approach suffers several shortcomings For example, the variation in the diameter of the cells will inherently result in more wasted or void area between the cells, and, therefore, the density of cells on the transducer or the effective vibrating surface of the transducer will not change. In practice, this approach as applied to circular shaped CMUT cells has no significant impact on the acoustic output of the device and will, at best, only affect the resonant frequency of the transducer.
With regard to the discussion above regarding transducer design trade-offs and the behavior of ultrasonic transducers using capacitive membranes (also referred to as cells) as vibrating elements, it will be appreciated that the cells of CMUTs must be carefully tailored to produce the final characteristics desired. In practice, a major task for a designer will thus be the determination of a suitable cell geometry and behavior (i.e., stiffness) in order to provide the CMUT with the desired electrical and acoustical characteristics. In doing this, an optimized cell structure has to first be determined and thereafter this structure has to be repeated over an area of the substrate so as to provide an elemental transducer.
It is also well known that shaping of the cells is essential in the optimization of transducer surface mapping. In this regard, membranes or cells that are shaped as polygons or rectangles are better suited for minimizing the non-functional area of the substrate. As aforementioned, the silicon substrate is generally populated with thousands of CMMs (Capacitive Micromachined Membrane) which are organized in small groups which are connected together. Thus, each group of CMMs forms an elemental transducer (transducer element). A suitable interconnection means is then optionally provided at the sides of the transducer to facilitate further assembly operations. Since the silicon substrate is set in wafers, and the processing cycles are long time consuming tasks, each wafer of the substrate will be fully patterned with transducer masks in order to reduce costs and processing time and so that the mapping of the substrate surface can be carried out with different transducer designs, as required.
Turning to CMUT array construction and manufacture, the elemental transducers are formed as a combination of a plurality of shunted CMMs. These transducer elements are separated from each other by a kerf or small space that physically isolates the adjacent elements. The kerfs are made as narrow as possible to prevent the loss of sensitivity but should be of such a width as to provide an adequate acoustic barrier against acoustic cross talk between adjacent transducer elements. Earlier CMUT devices that have been shaped to form arrays were not diced, and measurements carried out with respect to such devices have demonstrated that the bulky silicon kerfs employed provide a very weak acoustic barrier so that the image quality provided by such devices is quite inferior to that of standard transducer devices.
Further developments in CMUT construction include improvements in the transducer characteristics provided by implementing dicing, and using polymer filled kerfs in combination with high loss backing members to better meet the requirements of high quality beamformers. Dicing of the CMUT device will further provide the device with the ability to bend longitudinally. This enables the formation of curved arrays of the type that are in widespread use in medical applications.
In other publications to Ladabaum et al., Sensant Corp. (Curved Micromachined Ultrasonic Transducers, K. A. Wong, s. Panda and I. Ladabaum; IEEE-UFFC Symposium 2003), the authors disclose another technique for bending the substrate, including a step of thinning the material from the back side thereof by grinding of the substrate. The grinding operation is carried out through the thickness of the substrate to such an extent as to impart flexibility. In this regard, the transducer is made sufficiently flexible to uniformly conform to a desired radius of curvature. This process is limited by the great fragility of the substrate after the grinding operation, thus making all further manufacturing operations more delicate than with conventional techniques.